1. Field of the Invention
The invention relates to an apparatus for the contactless guiding of webs of material having at least two blow nozzles, designed as slits or rows of holes, arranged one behind the other in running direction of the web of material and extending transversely to it. The blow jets of the nozzles are guided in the same direction and directed obliquely against the web of material and on the side towards the web of material by flow baffles, of which the flow baffle adjoining the rear blow nozzle in running direction of the web of material is curved convexly toward the web of material.
Such bearing surfaces with a flow passing over them form together with the web of material Venturi diffusers, the web of material being made to flutter by the Bernoulli effect. The fluttering can be suppressed by special measures, for example by a perforation in the bearing surface, as then additional air flowing in allows the flow to remain on the web of material, or by bearing surface suction removal, as then the flow remains only on the bearing surface. Such one-sided flows are also produced in exceptional cases if the tension of the web of material and the material weight on the one hand and the shape of the bearing surface and the flow passing over the bearing surface on the other hand are matched. However, the need for cleaning and freedom of movement in the production of a paper machine generally neither permits the above measures nor allows the conditions for specific flow circumstances to be maintained. As with a Venturi diffuser, the narrowest point between the web of material and the back of the bearing surface changes depending on the conditions, it is namely not possible for fluttering to be avoided and there is no certainty of preventing contact.
2. Discussion of Prior Art
In the case of a known apparatus of the type mentioned at the beginning, the flow baffle that runs between the convexly curved flow baffle of the rear blow nozzle and the front blow nozzle, substantially parallel to the web of material. To make the flow become turbulent, the surface of the parallel-running flow baffle may be roughened, in particular may have corrugations. No special precautions against fluttering of the web of material are taken. There is therefore the risk that the web of material will hit something (DT 19 54 880 B2).
The risk that webs of material tend to flutter over freely guided sections is particularly great at high running speeds. Investigations have shown that fluttering takes place due to flows at the edge of a material web transverse to the running direction, in areas of relatively high and low pressure. Fluttering subjects the web of material to high stresses at the edges, so that there is the risk of the web of material tearing. On paper machines having upper and lower drying cylinders, this risk exists at the freely guided sections between the upper and lower drying cylinders. At these points, there is also the difficulty of carrying away the clouds of steam produced during drying, in order that the paper web can be dried evenly over the full machine width. It has been attempted to come to grips with the fluttering problem by a disposable felt running along with the paper web over the free sections and guiding it. However, the ventilation of the felt necessary for even drying brings new problems with it.